The invention is based on an apparatus for monitoring a system for delivery of secondary air to the exhaust gas of an internal combustion engine. It has already been proposed (German Patent Application P 41 41 946.4, corresponding to U.S. patent application Ser. No. 07/993,004) to supply secondary air to the combustion products on the exhaust side, in order to reduce polluting exhaust components and to heat the catalytic converter. By way of example, such an apparatus includes a conduit system with a secondary air pump for pumping the secondary air flow and for building up a pumping pressure, a blocking valve for selectively switching the secondary air introduction on or off, and a check valve that prevents exhaust gas from reaching the blocking valve counter to the pumping direction of the secondary air pump. A control unit evaluates engine operating parameters, such as the lambda value, and controls the operation of the secondary air pump and the switching of the blocking valve.
If the function of such a system for delivering secondary air is to be monitored by components belonging to the system, then by way of example, with the blocking valve open and with the lambda control active, secondary air can be introduced into the engine exhaust line downstream of the lambda sensor, and the change in the lambda value can be utilized to evaluate the function. This procedure has the disadvantage, however, that during the monitoring process a relatively large quantity of secondary air is fed into the exhaust line, which causes a sharp change in the exhaust composition and hence worsens the conversion rate of the catalytic converter. Overheating of the catalytic converter can also occur.